The present invention relates generally to phosphor binders, and particularly to phosphor binders for use in a backlight for liquid crystal display (LCD) devices.
Obtaining the maximum light energy output for a given power input to a fluorescent lamp used as a backlight in an active matrix liquid crystal display (AMLCD) is an important operational feature in this type of display system. In particular, AMLCD devices transmit very little of the backlight provided. For a color AMLCD, only 2.5 to 4% of the backlight passes through the AMLCD. For monochrome applications, up to 12% of the backlight passes through the LCD. In either case, the most efficient use of backlight provided must be obtained to maximize the limited light passage capabilities of the AMLCD. The lumens (light out) per watt (light in) conversion in a LCD backlight system can be taken as a measure of efficiency for a fluorescent lamp backlight system. As may be appreciated, it is desirable to maximize the energy efficiency of a fluorescent lamp backlight system.
Light produced by a conventional fluorescent lamp is a result of excited phosphor exposed to ultraviolet (UV) light energy, e.g., generated from a mercury arc stream passing through a tube having phosphor bound on its inner surface. Fluorescent lamps, typically used in backlighting an LCD device, provide the best lumens per watt conversion efficiency relative to other practical light sources. Despite this highly efficient character of fluorescent lamps relative to other types of lighting devices, further improvement in the efficiency of LCD backlights is desired.
One aspect of efficient use of energy applied to a fluorescent lamp in an LCD backlight system requires use of as much of the UV light as possible in exciting the phosphor molecules to produce visible light.
Fluorescent coatings, in conventional fluorescent lamp manufacturing, result from a phosphor-binder slurry drawn into a glass tube, i.e., lamp envelope, then allowed to run out of the tube. The residual slurry material, i.e., that left on the interior walls of the glass tube, is refined through high temperature baking to remove binder material that would otherwise absorb UV light and cause a loss in light output, i.e., a loss in UV photons which could be otherwise used to excite the phosphor particles. The result of this phosphor coating process is a moderately uniform layer of phosphor on the inside of the tube. It is known in the industry that an ideal phosphor coating is on the order of 3 to 5 phosphor particles thick; the average phosphor particle size being in the micrometer (10.sup.-6) range. Excitation efficiency drops for coatings thicker than the optimum thickness because phosphor particles are not fully excited by the ultraviolet photon bombardment and light output falls drastically. Likewise, phosphor coatings thinner than the optimum thickness do not use all the potential light producing ability of the ultraviolet photons generated by the mercury arc stream. Light output is then less than that possible for the amount of power provided to the lamp in producing the arc. As used herein, the terms "relatively thin" and "relatively thick" presented in reference to a phosphor coating shall refer to the thickness of the phosphor coating as being either thinner or thicker, respectively, than the optimum phosphor coating thickness.
The prevailing rule for manufacturing fluorescent lamps is that a relatively thin phosphor coating is preferred and more practical than relatively thick phosphor coatings. High volume manufacturing processes generally will not support an optimum phosphor coating thickness.
A portion of the binding material can remain in the phosphor coating and absorb available UV light energy. The energy of the UV light absorbed by the residual binding material represents a loss or inefficiency because it does not contribute to phosphor excitation in production of visible light. Conventional fluorescent lamp backlight technology has used a lacquer-type of binder that must be baked out of the phosphor coating. If not fully removed, this can result in residue on the surface of the phosphor contaminating the arc stream and causing a loss in efficiency and shorter lamp life. Generally, the problem of residual phosphor binding material has not been particularly significant in conventional fluorescent lamps. The phosphor coating in a standard tubular glass fluorescent lamp can be baked at relatively high temperatures, e.g., 400.degree.-500.degree. centigrade, to remove virtually all of the organic binder material. This relatively high temperature baking step leaves little or no residual binder material, and therefore provides a process for eliminating loss due to UV light absorption by residual phosphor binding material.
When a fluorescent lamp device cannot be taken to such relatively high temperatures, i.e., substantially less than 400.degree.-500.degree. centigrade, a significant portion of the phosphor binding material would remain and present opportunity for absorption of UV light. Unfortunately, LCD backlight devices are desirably constructed of materials other than high-temperature resistent materials, e.g., desirably constructed from plastic material. Accordingly, conventional manufacturing materials and processes for establishing a phosphor coating are not acceptable with respect to use in such LCD backlights.
Ultraviolet photons are easily absorbed by most materials considered suitable for use as a binder for phosphor coatings in fluorescent lamps. Absorption of UV energy that otherwise has the potential to make visible light results in a significant loss in efficiency of fluorescent lamps regardless of their shape and construction. It is desirable, therefore, that a binder material be provided for fluorescent lamps which binds phosphor particles to a surface, but does not require high temperature baking to remove residual material yet still absorbs little or no UV light energy. UV light energy is then applied efficiently to the excitation of phosphor particles to produce visible light and thereby enhance the overall efficiency of the LCD backlight.